Cells
Cells is the A-level Biology topic covering microscopy, the structure of eukaryotic and prokaryotic cells, cell fractionation, the cell cycle (mitosis) and the fluid mosaic model of the cell membrane. It builds directly on biological molecules and is assessed with calculation-heavy magnification questions. Required practicals include microscopy of cells and investigating membrane permeability.
Method
- Learn the magnification formula, image size = actual size x magnification, and rearrange it for whichever value is missing, always converting to the same units first (usually micrometres).
- Compare prokaryotic and eukaryotic cells feature by feature (nucleus, ribosome size, plasmids, membrane-bound organelles) rather than trying to memorise two separate lists.
- For organelle-function questions, link structure directly to function, for example folded cristae in mitochondria increase surface area for ATP synthase.
- Learn the order of the cell fractionation method (cold, buffered, isotonic homogenisation, filtration, then ultracentrifugation) and explain each condition in terms of protecting or separating organelles.
- For mitosis questions, learn the order of the phases (prophase, metaphase, anaphase, telophase) and one distinguishing feature of each, then practise calculating a mitotic index from cell counts.
- For membrane questions, describe the fluid mosaic model precisely (phospholipid bilayer, embedded/intrinsic and extrinsic proteins, cholesterol) before explaining how a factor changes permeability.
Worked example
A student calibrated an eyepiece graticule against a stage micrometre. At a given magnification, 25 eyepiece units aligned exactly with 0.50 mm on the stage micrometre. The student then measured a plant cell as 12 eyepiece units wide. Calculate the actual width of the cell in micrometres.
- Convert the stage micrometre reading to the same unit as the final answer: 0.50 mm = 500 micrometres.
- Find the value of 1 eyepiece unit: 500 / 25 = 20 micrometres per eyepiece unit.
- Multiply this value by the cell's measured width: 12 x 20 = 240.
- Final answer: the cell is 240 micrometres wide.
Practice questions
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Q1State one structural feature present in a eukaryotic cell but absent from a prokaryotic cell.Show answer
Answer: A membrane-bound nucleus (also accept membrane-bound organelles such as mitochondria).
Q2Name the organelle responsible for modifying and packaging proteins before they are secreted from the cell.Show answer
Answer: The Golgi apparatus (Golgi body).
Q3State the correct order of the stages of mitosis.Show answer
Answer: Prophase, metaphase, anaphase, telophase.
Q4In the context of microscopy, what is meant by the term resolution?Show answer
Answer: The minimum distance between two points at which they can still be distinguished as two separate points.
Q5A photomicrograph of a cell measures 60 mm across. The actual width of the cell is 15 micrometres. Calculate the magnification of the image.Show answer
Answer: x4000 (convert 60 mm to 60000 micrometres, then 60000 / 15 = 4000)
Q6In a root tip squash, 320 cells were viewed in total, of which 24 were undergoing mitosis. Calculate the mitotic index of this sample, to 2 significant figures.Show answer
Answer: 0.075 (24 / 320 = 0.075)
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
Explain how the magnification of a microscope differs from its resolution, giving two distinguishing points.
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Cell fractionation is used to separate organelles for study. Explain why the solution used during homogenisation must be (i) ice-cold, (ii) isotonic and (iii) buffered, and state the general order in which organelles are separated by ultracentrifugation.
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A group of students investigated the effect of temperature on the permeability of beetroot cell membranes. Beetroot cylinders of equal size were placed in water baths at 10, 25, 40, 55 and 70 degrees C for 10 minutes, then the absorbance of the surrounding water (due to leaked red pigment) was measured with a colorimeter. Absorbance increased only slightly between 10 and 40 degrees C, then increased sharply between 40 and 70 degrees C. Explain these results, and state two variables other than temperature that should have been controlled.
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Free printable worksheet
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